In recent years, there has been a growing use of a laser beam in an extremely wide range in machining processes for manufacturing products and components or in some other processes. This laser beam is a specific coherent light of which amplitude and a wavelength are identical, so that the laser beam is differentiated from an incoherent light such as a natural light of which amplitude and a wavelength are different from each other.
There is a method, as one of laser machining methods based on the conventional technology, of irradiating each hole with a prespecified number of pulses according to a preset oscillation frequency when a plurality of holes are to be machined by means of using a laser beam comprising a plurality of pulses. This method is referred to as "Mode A" in this specification hereinafter.
There is also a method, as another laser machining method different from that described above, of irradiating laser beam pulses to all holes in a scanned area shot by shot, going back to the first hole after all the holes therein have been irradiated once, irradiating again laser beam pulses from the first hole shot by shot, and repeating the steps to a preset number when using the galvano scan mirror. This method is referred to as "Mode B" in this specification hereinafter.
FIGS. 15A and 15B show cross-sectional views when via-holes are machined by using a laser beam to a printed board, FIG. 15A is a view showing an insulating layer of a printed board 50 made from a resin, and FIG. 15B is a view showing an insulating layer of a printed board 62 made from an inorganic basic material impregnated with a resin.
In the figures, the printed board 50 or 62 has lands 54 for connection located on a core circuit board 52 thereof, an insulating layer is formed with an epoxy resin 56 or a glass epoxy resin 64 with glass cloth 66 mixed therein on the lands, then via-holes 58 machined with a laser beam are formed thereon, and a whole surface thereof is subjected to pattern plating 60.
As described above, the via-holes 58 formed on the printed boards 50 and 62 shown in FIG. 15A and FIG. 15B can be formed by using either one of laser machining methods of Mode A and Mode B. Especially, when the insulting layer is made from a resin as shown in FIG. 15A, there is no difference in the machining quality between Mode A and Mode B.
Therefore, machining is generally executed by using Mode A with making an oscillation frequency higher because high productivity and time reduction can be obtained in this mode. The phrase of making an oscillation frequency higher (high frequency) herein means setting the oscillation frequency equal to or higher than a frequency obtained from an inverse number of a time required for positioning a plurality of holes. If the frequency is set to a lower value than the above value, the productivity may become lower in Mode A as compared to that in Mode B.
Next description is made for the concept of machining time as described above with reference to a galvano-mirror used as an example thereof. FIG. 16 shows a diagram showing a relation between the number of pulses and machining time for each of oscillation frequencies in Mode A and Mode B. The figure shows a galvano scan that can decide a relation between an oscillation frequency and machining time and position within 2.5 msec. Because the galvano scan is capable of positioning within 2.5 msec, it means that the required oscillation frequency is 400 Hz obtained from an inverse number of the required time.
As shown in FIG. 16, assuming that the oscillation frequency is 400 Hz, the value matches a frequency (cycles of repetition) for galvano scan, so that Mode A and Mode B are equivalent so far as the machining time is concerned. If the oscillation frequency is lower than 400 Hz (the case of 200 Hz in FIG. 16), the machining time required in Mode A is longer than that required in Mode B. If the oscillation frequency is higher than 400 Hz (the case of 800 Hz in FIG. 16), the machining time required in Mode A is shorter than that required in Mode B.
However, in the laser machining method based on the conventional technology, as shown in FIG. 15B, it has been understood that there occurs a big difference in machining quality between Mode A and Mode B when a resin impregnated with an inorganic substance such as glass cloth or ceramics is used as a basic material for an insulating layer of the printed board.
At present, in the galvano scan system generally used when holes are to be machined on a printed board by means of using a carbon dioxide laser, a time required for moving and positioning a laser beam by a galvano-mirror is short with the frequency equivalent to 400 Hz at the highest speed. And for this reason, in Mode A described above, no merit is obtained in the productivity unless the frequency is set to a value higher than 400 Hz. However, in glass epoxy resin 64 shown in FIG. 15B, for example, a decomposition temperature of epoxy is about 600 K (327.degree. C.) while a transition temperature of glass is about 1100 K (827.degree. C.).
When materials each having a significantly different thermal feature from each other as described above are used to form an insulating layer, if the oscillation frequency of a laser beam is made higher, a time of reaction between a laser and materials becomes longer, so that the epoxy portion is selectively decomposed and removed relatively more than that of the other material. For this reason, glass fiber is protruded from a wall surface of the machined hole, so that pattern plating 60 cannot be formed with sufficient smoothness on the surface thereof, whereby the machining quality is deteriorated disadvantageously.
In contrast, when the laser machining method in Mode B described above is combined with the galvano scan system, the larger number of holes are to be machined in a galvano area, a time interval between a point of time when a hole is irradiated and a point of time when the same hole is irradiated again becomes longer. In other words, the machined material is cooled down more in proportion to the time interval, so that the interactive time is reduced, and such a phenomenon that epoxy resin is selectively decomposed and removed hardly occurs, which allows better machining quality to be obtained. However, when holes are machined in Mode B, when a plurality of pulses are irradiated per one hole, a longer machining time is required excluding a case where machining can be performed by one pulse per one hole, thus the productivity in Mode B becomes remarkably lower as compared to that in Mode A.